1. Field of the Invention
The present invention generally relates to electro-optical systems for reading indicia, for example, bar code symbols, having parts with different light reflectivities and, in particular, to an arrangement for, and a method of, improving the performance of data collection.
2. Description of the Related Art
Various electro-optical readers and systems have previously been developed for reading bar code symbols appearing on a label, or on a surface of a target. The bar code symbol itself is a coded pattern of indicia. Generally, the readers electro-optically transform graphic indicia of the symbols into electrical signals which are decoded into alphanumeric characters. The resulting characters describe the target and/or some characteristic of the target with which the symbol is associated. Such characters typically comprise input data to a data processing system for applications in point-of-sale processing, inventory control, article tracking and the like.
The specific arrangement of symbol elements, e.g., bars and spaces, in a symbol defines the characters represented according to a set of rules and definitions specified by a code or symbology. The relative size of the bars and spaces is determined by the type of code used, as is the actual size of the bars and spaces.
Electro-optical readers have been disclosed, for example, in U.S. Pat. No. 4,251,798; U.S. Pat. No. 4,369,361; U.S. Pat. No. 4,387,297; U.S. Pat. No. 4,409,470, U.S. Pat. No. 4,760,248 and U.S. Pat. No. 4,896,026, all of which have been assigned to the assignee of the present invention. These readers generally include a light source consisting of a gas laser or semiconductor laser for emitting a light beam. The laser beam is optically modified, typically by a focusing optical assembly, to form a beam spot having a certain size at a predetermined target location.
The light beam is directed by a scan component along a light path toward a target symbol. The reader operates by repetitively scanning the light beam in a scan pattern, for example, a line or a series of lines or scans across the target symbol by movement of the scan component such as a mirror disposed in the path of the light beam. The scan component may sweep the beam spot across the symbol, trace a scan line across and beyond the boundaries of the symbol, and/or scan a predetermined field of view.
Readers also include a data collection circuit having a sensor or photodetector which functions to detect light reflected or scattered from the symbol. The photodetector or sensor is positioned in the reader in an optical path so that it has a field of view which extends across the symbol. A portion of the light beam reflected from the symbol is collected and detected and converted into an analog electrical signal. A digitizer digitizes the analog signal. The digitized signal from the digitizer is then decoded by a decoder based upon the specific symbology used for the symbol. A microprocessor controls the overall operation.
The data collection circuit will, however, not be able to readily read, if at all, the symbol if the symbol is poorly printed or damaged. In the prior art, a single data collection channel having a digitizer circuit operative at a single threshold is typically employed to digitize the analog signal. However, if the threshold is set too high, a high level of noise resulting from printed imperfections may be thresholded out, but at the risk of thresholding out a weak signal for an actual element of the symbol. If the threshold is set too low, noise may be detected as data elements of the bar code. Hence, a single threshold is not well suited to enable a reader to readily read symbols of different print qualities, at different distances from the reader, of different symbol densities, and under different real world conditions. It is known to switch thresholds on successive scans in order to enable the reader to read different kinds of poorly printed symbols. However, it often takes several scans before a workable threshold is selected for a particular symbol being scanned. As a result, although the reader will ultimately be able to read the symbol, the reader will feel sluggish.
U.S. Pat. No. 6,497,366 discloses a dual channel data collection circuit in which light reflected from a bar code symbol is converted to a-raw analog signal, and then processed in two independent data collection channels. Each channel has its own separate preamplifier, bandpass filter, threshold generator and analog-to-digital converter. Each scan produces two output signals, with aggressive thresholding on one channel, and with less aggressive thresholding on the other channel. Reading terminates when a processor determines that an output signal is decodable.
As advantageous as this dual channel circuit is in collecting data, each scan produces the same two output signals. The characteristics of the filter and the threshold generator, for example, do not change from one scan to the next, and their default values may not be adequate to allow the symbol to be read. In addition, there is much extra cost and complexity involved in providing two identical collection channels with the same components.